1. Field of the Invention
The invention is generally related to the area of integrated circuits designs. More particularly, the invention is related to designs of multiple inductors or transformers in integrated circuits, wherein the transformers have strong magnetic couplings but do not occupy multiple areas. The invention is also related to transformers with one or more center-taps.
2. The Background of Related Art
An inductor is a passive electronic component that stores energy in the form of a magnetic field. In its simplest form, an inductor consists of a wire loop or coil. The inductance is directly proportional to the number of turns in the coil. Inductance also depends on the radius of the coil, the space between the turns, thickness of the material of the coil, and on the type of material around which the coil is wound. For a given coil radius and number of turns, dielectric materials such as wood, glass, and plastic result in the least inductance while ferromagnetic substances such as iron, laminated iron, and powdered iron increase the inductance. The shape of the core as well as the wire or coil can also be significant. The standard unit of inductance is the henry, abbreviated H. This is a large unit. More common units are the microhenry, abbreviated μH (1 μH=10−6 H) and the millihenry, abbreviated mH (1 mH=10−3 H). Occasionally, the nanohenry (nH) is used (1 nH=10−9 H). As signal frequency goes high, for example, in gigahertz range, inductors with the picohenry (pH) are often used (1 pH=10−12 H).
Inductors are used with capacitors in various applications such as wireless communications. An inductor connected in series or parallel with a capacitor can provide discrimination against unwanted signals. When two or more inductors are arranged close to each other, transformers are created. In addition to the inductances, performance of the transformers is also controlled by coupling factors that are in turn determined by the proximity of the inductors. In a simple form, a transformer has two inductors, usually with an iron core, that have two lengths of wires wrapped around it. The two coils of wire do not electrically connect, and are normally attached to different circuits. Inductors and transformers have been widely used and can be found almost in every electronic circuit or system.
However, it is well known that it is difficult to fabricate inductors or transformers onto integrated circuit (IC) chips. To have usable inductors (or transformers) in IC chips, the wafer areas occupied by the inductors can be significant, resulting in very expensive IC chips. In some cases, resistors are substituted for inductors. In other cases, inductance is simulated by simple electronic circuits using transistors, resistors, and capacitors fabricated onto IC chips. As the frequency of signals goes higher, the performance of such inductor “substitutions” are no longer satisfactory. True inductors or transformers are demanded.
FIGS. 1A–1D show respectively four transformers that are commonly used in silicon, each of the transformers including two inductors wound according to one type of winding configuration. It can be understood that, when several such transformers are used in an IC chip, multiple spaces for the transformers must be allocated, resulting in a very expensive IC chip, because the cost of an IC chip is largely determined by the physical area of the IC chip in a piece of semiconductor wafer. Essentially, the larger a physical area of an IC chip is, the higher cost the IC chip will be.
There is thus a tremendous need for solutions of providing on-chip transformers without taking up too much wafer space and, at the same time, having strong couplings to provide high secondary inductance values.